Microelectromechanical systems (hereinafter MEMS) have been developed for movable devices such as hygroscopes, accelerometers, tunable RF capacitors, digital mirrors, sensors and the like. They are used for forming electrical and mechanical structures on a substrate, particularly a substrate of silicon or silicon-containing material. They are made using conventional semiconductor processing techniques, such as chemical vapor deposition and plasma etch for example.
During fabrication, the devices are formed from a layer of material, part of the substrate, which is partially, but controllably, etched away to release portions of the device from the substrate so as to form movable parts. The device however can remain anchored to the substrate after its release. The etched, or sacrificial layer, is suitably a silicon oxide.
FIG. 1 illustrates a simple three-layer substrate that can be used to make a MEMS device. The substrate in a cross sectional view, includes a layer of silicon 10 covered with a layer of silicon oxide 11 and a layer of polysilicon 12 thereover. The layer of silicon oxide 11 is to be etched to release or suspend the layer 12 above the silicon substrate 10.
FIG. 2 is a cross sectional view of an embodiment of the invention that illustrates a released polysilicon device. A lever 14 is released, or partially separated from, a silicon substrate 16, except for a connection or anchor 18. The lever 14 is free to move up and down with respect to the substrate 16 after actuation, as by an electric signal. A layer of silicon oxide 19, shown in phantom, has been partially etched away to release or separate the lever 14 from the substrate 16.
Release is a complex process wherein the silicon/silicon oxide material is controllably removed, or etched away. If too much material is removed, the desired structure is undercut, so it is no longer anchored to the substrate. On the other hand, if too little material is removed, frozen structures are formed that are not able to move as intended.
An example of a more complex MEMS device is shown in FIG. 3. FIG. 3 is a top view of two orthogonal linear drives 30 that are linked to a rotary gear 32. When properly released, the rotating gear 32 has unlimited movement, and can revolve in excess of 350,000 revolutions per minute (rpm). This device has been demonstrated to have a lifetime of over 7×109 revolutions with millions of start/stop cycles.
Different materials have different problems that are encountered during the release etch process. This is illustrated in FIG. 4, which is a graph of etch rate, in angstroms per second, versus etch time in seconds, for an aqueous hydrogen fluoride etch of different materials. Line A is the etch rate for doped CVD silicon oxides, such as borophosphosilicate glass (BPSG), phosphosilicate glass (PSG), and borophosphotetraethoxysilane (BPTEOS). The process time increases rapidly to a fairly stable etch rate. Line B however, which is illustrative of the etch rate for dense oxides, such as thermal oxide, TEOS and high temperature deposited oxides (HTO), shows a long initiation time, of almost 5 seconds, and then only a slow increase in the etch rate. It is apparent that there is an initiation time for dense silicon oxides that is not found for doped oxides. Further, the etch rate is much higher for doped CVD oxides. As is known, etch rates also vary with device design.
An aqueous HF etch solution, or an HF bath, is very easy to make, easy to use, and it is inexpensive. Thus an aqueous solution of HF has traditionally been used to isotropically etch silicon oxides. However, the surface tension of the solution causes capillary forces to pull the micro-sized structures together, and causes what is known as stiction, or sticking together of the etched parts so they are not able to move freely.
The initiation time, as discussed above, also varies with the amount of water present in various silicon oxides. Thus it is very difficult to determine just when etching begins using aqueous HF, and to determine the time one needs to etch in order to obtain release, but not total separation of, a device from its substrate.
Using aqueous HF as the etchant, FIG. 5, which is a graph of the amount of material removed by the etchant versus the time etching is continued, in seconds, for different silicon oxides. Line A shows the removal rate for a first group of doped CVD silicon oxides, such as PSG. Line B shows the removal rate of silicon oxide deposited from TEOS, and line C shows the removal rate of dense TEOS oxide. PSG etches much faster than dense TEOS oxide using aqueous HF as the etchant, while TEOS oxide has an intermediate etch rate. Thus 5000 Å of PSG can be removed in one minute, while only 1000 Å of dense TEOS oxide is removed in one minute.
In view of the above problems of aqueous HF, anhydrous HF, which is also an excellent isotropic etchant, has also been considered. However, it is a very strong acid, and thus attacks materials from which an etch chamber is generally made, and its fumes are dangerous. Thus this etch must be carried out in a suitable chamber, one that is at least partly impervious to strong acids. The advantage to using anhydrous HF however is that the liquid-solid phase does not exist as it does in an aqueous HF solution, and capillary forces that cause stiction are by-passed, greatly reducing the amount of stiction caused during the etch.
However, anhydrous HF has the same problems of variation of etch rates and initiation times as does aqueous HF.
Various methods have been tried to determine the actual amount of time required to release particular devices from particular substrates. Mass spectrophotometers have been used to identify substrates and the amount etched in a given time period, but they can only be used at low pressures. Optical microscopes have been used to monitor etching in real time, but process conditions can interfere with proper viewing.
Thus a means for monitoring the etch rate, and thus the time to release, of a device from its substrate, in real time, using anhydrous HF, has been sought. The etch monitor and method of use for silicon-containing materials must monitor the etch so it can be carried out in a highly controllable way, one that avoids both underetching and overetching, and that avoids stiction when forming MEM structures.